Frontline had an excellent piece on the (lack of) reliability behind most of crime forensics. Fingerprints in particular are mentioned as being very unreliable and unscientific. The only scientifically rigorous piece of "CSI" is DNA matching.
http://mobile.nytimes.com/2013/09/17/science/dna-double-take...
In addition to that, there are problems with bias and statistical independence. A given marker is unlikely to be present in exactly 50% of the population, and to the extent that it isn't it can reduce the probability by that amount that a test match is a true match. Meanwhile the suspect pool for a given crime is likely to encompass several (perhaps many) members of the same extended family, who for the obvious reason are significantly more likely than random members of the world population to have the tested markers match one another. Even within a city you will generally see concentrations of specific ethnicities who may have a higher statistical incidence of specific genetic markers than other populations, which can screw up the numbers by an amount that historically hasn't even knowable because we don't have good numbers on the statistical incidence of specific markers within geographical populations.
The place where this is most pernicious is when they get a sample from a crime scene and run it against some "DNA database" to find a hit. Then everybody is talking about the probability that X suspect would match the DNA at the crime scene rather than the probability that someone in the database would match even if the actual perpetrator wasn't in the database.
Or the probability that, match or not, the DNA from the crime scene belongs to the criminal.
And not, say, someone the victim came in contact with earlier, someone that happened to be in the crime scene before the crime took place, or even some third guy the actual criminal took a DNA from in order to frame him.
I thought that was pretty much always the case. Which is why DNA evidence is never used alone - you don't take DNA traces found at a scene of crime, run it against a huge database, find a match, close the case and try and sentence the matching person.
Instead you either investigate whether that matching person had means and motive and no alibi, or (more often) you check the DNA only against people you already suspect for whatever reasons.
Both variants reduce the likelihood of false positives by quite a few orders of magnitude.
As one example, combine a fairly limited set of targets with gel chromatography, and varying quality/accuracy of analysis/analysts of same... And you have a lot less "uniqueness" than things like the common, public term "DNA fingerprint" imply.
Yes, it may be a useful tool in combination with proper understanding of its limitations. However, we have (in the U.S., for example) and adversarial judicial process and prosecutors have been shown to often not place such understanding even in context let alone as a primary concern. If the defence is lacking, including simply financially to engage its own "expert witnesses"... misbegotten interpretations can and do rule the day.
Yes, this really happened - at least once that we know of: https://en.wikipedia.org/wiki/John_Schneeberger
During his 1999 trial, Schneeberger revealed the method he used to foil the DNA tests. He implanted a 15 cm Penrose drain filled with another man's blood and anticoagulants in his arm. During tests, he tricked the laboratory technician into taking the blood sample from the place the tube was planted.
Then again, I guess we've seen that you literally cannot be too paranoid.
You don't have to have "everyone's DNA on file". It's actually pretty trivial even for your neighbor or whoever to get your DNA.
As for the police falsifying evidence, there's a wikipedia-long history of cases, in Europe, Latin America, Asia, etc. Especially in politically charged times, like the sixties and seventies. Heck, something like half of Italy's government in the 70's have been proved in later Italian courts to be involved in such things.
Sorry, I wasn't clear. I can dump a gallon of your blood and semen onto a dead guy in an alley, but how would the government trace that blood and semen back to you?
Well, as the culprit, you can always arrange some things or leave other stuff that also points to me.
Also, your main benefit is that the police will more easily believe that it wasn't you (since your DNA won't match).
http://www.nytimes.com/2013/09/17/science/dna-double-take.ht...
> But scientists are finding that it’s quite common for an individual to have multiple genomes. Some people, for example, have groups of cells with mutations that are not found in the rest of the body. Some have genomes that came from other people.
> Women can also gain genomes from their children. After a baby is born, it may leave some fetal cells behind in its mother’s body, where they can travel to different organs and be absorbed into those tissues. “It’s pretty likely that any woman who has been pregnant is a chimera,” Dr. Randolph said
It's much less common than a consumer-grade scanner and some wood glue.